vfio/ccw: get irqs info and set the eventfd fd

vfio/ccw: get irqs info and set the eventfd fd

vfio-ccw resorts to the eventfd mechanism to communicate with userspace.
We fetch the irqs info via the ioctl VFIO_DEVICE_GET_IRQ_INFO,
register a event notifier to get the eventfd fd which is sent
to kernel via the ioctl VFIO_DEVICE_SET_IRQS, then we can implement
read operation once kernel sends the signal.

Reviewed-by: Eric Auger <[email protected]>
Acked-by: Alex Williamson <[email protected]>
Signed-off-by: Dong Jia Shi <[email protected]>
Message-Id: <20170517004813[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>